Softfail div by zero

It passes all spectests after the following changes:

* remove the test from dollar_bang.t that uses try { 1 div 0 } to set $! and then explodes when trying to access $!.defined (is this perhaps a bug with soft exceptions?)
* skip two more tests in S32-num/rat.t where it expects (3/0).num to be Inf and (-42/0) to be -Inf. Those should wait for Inf, -Inf and NaN to become soft failure objects.